1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to a wireless communication technology, and more particularly methods and apparatuses consistent with the present invention reduce power consumption of a wireless LAN device operating in a wireless network.
2. Description of the Related Art
With the development and spread of digital products, corresponding digital technologies require development of high-speed wireless LANs (local area networks) that support a transmission rate of higher than 100 Mbits per second. A multiple-input multiple-output (MIMO) technology is considered to be one of the candidate high-speed technologies of the next-generation wireless LAN that can satisfy the development requirement.
Work is proceeding on the IEEE 802.11n standard which has a purpose of applying a channel bonding technology, among other purposes, which makes it possible to use twice the current maximum bandwidth by using multi-channels together with MIMO technology.
IEEE 802.11n is a standard enabling a transmission speed of at least 100 Mbps at a MAC data service access point (MAC SAP) by changing the existing IEEE 802.11 PHY and MAC. Some schemes of the new standard that support a high-speed transmission speed aim at operating in association with the existing 802.11a or 802.11g wireless LAN systems. In order to achieve frequency efficiency at an unlicensed bandwidth, a practical model that requires the highest transmission support in supporting the transmission speed should attain at least 3 bits/sec/Hz at a PLCP service data unit (PSDU) level.
The currently developing IEEE 802.11n standard seeks to achieve a high data transmission speed at a MAC upper layer, a communication coverage distance, overall network capacity, power consumption, frequency band flexibility, an implementation cost and complexity, a compatibility with the existing wireless LAN standard, and a coexistence with the existing devices.
Products have been released that support these aims using an orthogonal frequency division multiplexing (OFDM) or MIMO technology. It is expected that diverse wireless LAN devices, such as wireless TV sets, wireless set-top boxes, wireless portable devices, wireless notebook computers, and others, will form a network in the unit of a wireless cell.
In order for such diverse wireless LAN devices to operate in a high-speed wireless LAN environment at any time and in any place, a stable power supply is required. Since the wireless LAN devices are portable, in most cases they are not supplied with a stable power like wired devices, and generally operate using compact and light rechargeable batteries.
FIG. 1 is a view illustrating an example of a wireless LAN network composed of a plurality of high throughput devices (HT devices). Referring to FIG. 1, handheld devices 2a and 2a among the HT devices have lower capability than other HT devices 1a, 1b, and 1c, due to the limited number of antennas provided thereto. Accordingly, the handheld device may not decode a certain frame when it communicates with other HT devices.
Nevertheless, the handheld device may operate in a reception mode with respect to the non-decoded frame, and this causes battery-power consumption to be increased. The respective HT devices show a significant difference in power consumption, depending on their operational states (e.g., a transmission mode, reception mode, and standby mode). The HT device consumes a lot of power in the reception mode compared to the transmission mode. Since the battery lifespan of the handheld device is limited, such power consumption causes a serious problem.
The power consumption occurring during the communication between the conventional HT devices will be described in detail with reference to FIGS. 2 and 3. It is assumed that, as shown in FIG. 2, the HT device 1 transmits data to the HT device 2, and receives an acknowledgement (ACK) from the HT device 2. The HT device 1 can transmit the data to the HT device 2 when the DIFS (Distributed coordination function Interframe Space) elapses from the time when a channel enters an idle state (assuming that a backoff timer of the HT device 1 is set to “0”). In this case, both the HT device 2 and the HT device 3 receive the transmitted data because of the nature of wireless communication. Although the target of the data is the HT device 2, the HT device 3 does not know that it should not receive the data until the data is processed in the MAC layer, and thus it continuously receives the data. In the same manner, the HT device 3 also receives the ACK that is transmitted when the SIFS (Short InterFrame Space) elapses after the data transmission is completed. Then, the HT devices can transmit the data after the DIFS in accordance with a backoff algorithm.
FIG. 3 is a view illustrating the power consumed by the respective HT devices during their communication operation as shown in FIG. 2. In the first DIFS period (period {circle around (1)})), all HT devices are in a listening mode, i.e., a mode for waiting to receive data. Then, while the HT device 1 transmits the data (period {circle around (2)}), the HT device 1 is maintained in a transmission mode (Tx mode), while other HT devices 2 and 3 are maintained in a reception mode (Rx mode).
Then, in an SIFS period (period {circle around (3)}), the HT device 1 reduces its power consumption by switching from Tx mode into Rx mode, while the HT device 2 increases its power consumption by switching from Rx mode into the Tx mode. In the meantime, the HT device 3 is maintained in the listening mode.
While the HT device 2 is in a period of transmitting the ACK (period {circle around (4)}), the HT device 2 is maintained in Tx mode, and other HT devices 1 and 3 are maintained in Rx mode. All HT devices are maintained in the listening mode in DIFS and backoff periods from the time when the channel is in the idle state.
As illustrated in FIGS. 2 and 3, in periods {circle around (2)} and {circle around (4)}, the HT device 3 is maintained in Rx mode, although it does not serve as a receiver. In particular, if the received data is non-decodable data, the HT device has been unnecessarily maintained in Rx mode.